1. Field of the Invention
The present invention relates to a position controlling device of a lens system with a variable focal length. Specifically, the present invention relates to a position controlling device for the focusing lens group of auto-focus cameras with a built-in inner focus-style or rear focus-style zoom lens.
2. Description of Related Art
In recent years, zoom lenses (lens systems with a variable focal length) and auto-focus functions are generally used on SLR cameras and video cameras. These zoom lenses can focus by scrolling the entire unit. Alternatively, zoom lenses using so-called rear focus methods or inner focus methods move or scroll one lens group among the movable lens groups that form the lens system. For these types of zoom lenses, the movement of the focusing lens group (the lens group that moves along the optical axis during the focusing action) is based on the photographic distance and the focal length of the lens system. Thus, the movement of the focusing lens group changes with focal length for the same photographic distance.
Therefore, most of these zoom lenses use a so-called stepped zoom lens where the focal length range is divided into multiple narrow focal length ranges and in which the focal length is not continuous. Stepped zoom lens methods determine the movement of the focusing lens group for the photographic distance at each focal length range.
The movement of the focusing lens group according to the photographic distance at each focal length range can be found by (1) recording a table of the movement amount of the focusing lens group that corresponds to a photographic distance or (2) recording the relationship of
For example, in Laid Open Japanese Unexamined Patent Application 62-284317, the movement amount of the focusing lens group at each focal length range for a given photographic distance is stored in advance in a memory. In Laid Open Japanese Unexamined Patent Application 5-113533, a fixed coefficient corresponding to each focal length range is preset in memory and the movement amount of the focusing lens group is calculated based on the photographic distance and the fixed coefficient.
However, in Laid Open Japanese Unexamined Patent Application 62-284317, required memory increases drastically when the focus precision is increased by increasing the stopping positions of the focusing lens group between the infinite focus position and the close-range focus position.
Also, in Laid Open Japanese Unexamined Patent Application 5-113533, the zoom lens focal length range is divided into multiple ranges, and a fixed coefficient is stored for each focal length range. This method is limited to specific focal length ranges that exist intermittently, or in other words, to stepped zoom lens functions. For such stepped zoom lens functions, if the number of zoom lens focal length range divisions is increased for zoom lenses with high zoom ratio, the number of fixed coefficients increases, and the required amount of memory must also increase.